The present invention relates to a novel process for forming a paint film, wherein a paint film is cured by allowing it to come into contact with a cross-linking agent.
Paint films using a thermoplastic resin are either soft and sticky at a high temperature or hard and brittle at a low temperature. Hence, paint films maintaining good physical properties from high to low temperatures should be cross-linked paint films.
However, heating to a high temperature is generally required for a thermal cross-linking reaction. Therein, enormous quantity of heat is required when the material to be coated has a large heat capacity; in many case, only 1% or so of the applied heat is used for the cross-linking reaction of the paint film itself. A cross-linking reaction at room temperature is desirable for effective use of the energy; however, such reaction is slow in general and does not proceed sufficiently.
A technique to make a cross-linking at room temperature proceed quickly will in general deteriorate the storage stability; thereby, pot life of the paint after the production is made short. In addition, the leveling period of the coated paint film is short, and the surface of the paint film turns coarse. In particular, this disadvantage is noticeable for the case of water-based paint and powder coating which do not use an organic solvent dissolving the resin. For the purpose of improving this defect, two package coatings are used; however, such coatings should be completed in a short period of time after the blending preparation of the paints to avoid an increase of the paint viscosity and gelation, which make the paint unusable.
In order to overcome these problems, forming a paint film, at first, that does not include cross-linking reactions and allowing enough leveling followed by curing by the contact of the paint film with a cross-linking agent will be satisfactory. Up to now, however, no process has been found that can form a practical cured paint film by combining a paint film with a cross-linking agent.